Indirect instruction predication

ABSTRACT

A method for selectively predicating instructions in an instruction stream by determining a first register address from an instruction, determining a second register address based on a value stored at the first register address, and determining whether to predicate the instruction based at least in part on a value stored at the second register address. Predication logic may analyze the instruction to determine the first register address, analyze a register corresponding to the first register address to determine the second register address, and communicate a predication signal to an execution unit based at least in part on the value stored at the second register address.

FIELD OF THE INVENTION

The invention is generally related to data processing, and in particular to processor architectures and execution units incorporated therein.

BACKGROUND

Instruction predication is a valuable feature in some processor architectures. Predication facilitates the prevention of execution of instructions in an instruction stream, which is referred to as “predicating” an instruction. Instruction predication is generally used in execution units performing algorithms that rely on loops and/or conditional branches and decision making. Instruction predication may be used, for example, in an algorithm utilizing a loop, where instructions implementing the loop are to be skipped when the loop is exited. As such, the instructions to be skipped when exiting the loop may be predicated in an instruction stream. In another example, a conditional instruction may have two possible outcomes, where instructions of one branch may be skipped depending on the resolution of the conditional instruction. As such, instruction predication logic predicates (i.e., prevents execution of) instructions corresponding to the branch not “taken” by the conditional instruction.

For example, image processing algorithms implemented in some three dimensional (3D) graphics applications incorporate a z-buffer algorithm test. In such 3D graphics applications, great care must be taken to avoid drawing objects that would not be visible, such as when an opaque object is closer to the camera than another object. In such a case, the object closer to the camera would block the farther object, and a 3D application that is attempting to draw this scene must not draw the further object. A z-buffer generally refers to a set of values that represent distance from the camera (sometimes called depth) for each pixel. Every time the rasterizing algorithm is ready to draw a pixel, it compares the depth of the pixel it is attempting to draw with the depth of the z-buffer for that pixel. If the z-buffer value indicates that the existing pixel is closer to the camera, the new pixel is not drawn and the z-buffer value is not updated. In contrast, if the new pixel to be drawn is closer to the camera, the new pixel is drawn and the z-buffer is updated with the new depth associated with that pixel. In a pixel shader of the 3D application, the algorithm may draw a pixel and update the z-buffer if the new pixel is closer to the camera than the older pixel stored in the z-buffer, but if the new pixel is not closer to the camera, the instructions following the z-buffer compare should be skipped and the next pixel should be tested. As such, predication may be utilized to skip instructions for a pixel depending on the outcome of the z-buffer compare.

In conventional processor architectures utilizing instruction predication, predication of an instruction is generally controlled by a state of a predication register. Each instruction in the instruction stream includes a predication register address portion corresponding to an address in the predication register, where the data stored at the register address indicates whether to predicate the instruction. As such, data of a predication register address may be adjusted to indicate whether to predicate an instruction, where the instruction will include data indicating the predication register address the processor may access to determine whether to predicate the particular instruction. For example, in the VLIW IA-64 processor architecture, a 64 bit predication register and 128 bit 3-instruction bundles are utilized, where each instruction includes a 41 bit instruction size and a predication field of 6-bits in the 41 bit instruction that determines which register address of the predicate register is used to determine whether to predicate the instruction.

However, in some fixed instruction length processor architectures, using bits of an instruction for a predication field uses up valuable space in the instruction that otherwise may be used for register addresses, opcodes, and/or other such data. As such, in some processor architectures, and particularly smaller fixed length instruction architectures, utilizing bits of an instruction for a predicate field may reduce the number of possible opcodes, source and/or target addresses that may be utilized in a processor using the architecture.

Therefore, a continuing need exists in the art for implementing instruction predication in processor architectures, and desirably without dedicating bits of an instruction to a predication field.

SUMMARY OF THE INVENTION

The invention addresses these and other problems associated with the prior art by selectively predicating instructions in an instruction stream using indirect instruction predication. In such embodiments, instructions in the instruction stream are predicated by determining a first register address in an instruction corresponding to an indirect predication register, determining a second register address based on the value at the first register address of the indirect predication register, and selectively predicating the instruction based on a value stored at the second register address.

Consistent with embodiments of the invention, a processing unit includes predication logic configured to determine whether to selectively predicate a respective instruction in an instruction stream in parallel with the instruction being decoded by decode logic of the processing unit. The predication logic determines a first register address from the respective instruction and determines a second register address from the value at the first register address. The predication logic determines whether to predicate the respective instruction based on the value stored at the second register address.

Therefore, in embodiments consistent with the invention, a first register address of one or more bits may be included in an instruction. The value at the first register address may point to second register address corresponds to a predication bit for the respective instruction. Based on the value at the second register address, the predication logic communicates a predicated instruction signal to an associated execution unit such that the respective instruction may be predicated based on the value at the second register address.

These and other advantages and features, which characterize the invention, are set forth in the claims annexed hereto and forming a further part hereof. However, for a better understanding of the invention, and of the advantages and objectives attained through its use, reference should be made to the drawings, and to the accompanying descriptive matter, in which there is described exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of exemplary automated computing machinery including an exemplary computer useful in data processing consistent with embodiments of the present invention.

FIG. 2 is a block diagram of an exemplary NOC implemented in the computer of FIG. 1.

FIG. 3 is a block diagram illustrating in greater detail an exemplary implementation of a node from the NOC of FIG. 2.

FIG. 4 is a block diagram illustrating an exemplary implementation of an IP block from the NOC of FIG. 2.

FIG. 5 is a block diagram illustrating an exemplary implementation of an IP block from the NOC of FIG. 2 or the processor of FIG. 1 incorporating predication logic suitable for implementing instruction predication consistent with embodiments of the invention.

FIG. 6 is a block diagram illustrating an exemplary implementation of predication logic from the IP block of FIG. 5 suitable for implementing instruction predication consistent with embodiments of the invention.

FIG. 7 is a flowchart illustrating a sequence of operations that may be performed by the IP block of FIG. 5 to selectively predicate instructions in an instruction stream.

FIG. 8 is a flowchart illustrating a sequence of operations that may be performed by the IP block of FIG. 5 to selectively predicate instructions in an instruction stream.

FIG. 9 is a block diagram of an example 32 bit instruction including support for instruction predication.

FIG. 10 is a block diagram of an example 32 bit instruction including support for indirect instruction predication.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of embodiments of the invention. The specific features consistent with embodiments of the invention disclosed herein, including, for example, specific dimensions, orientations, locations, sequences of operations and shapes of various illustrated components, will be determined in part by the particular intended application, use and/or environment. Certain features of the illustrated embodiments may have been enlarged or distorted relative to others to facilitate visualization and clear understanding.

DETAILED DESCRIPTION

Embodiments consistent with the invention selectively predicate instructions of an instruction stream by determining a first register address from a respective instruction, determining a second register address based on the value of the first register address, and determining whether to predicate the respective instruction based on the value of the second register address. In some embodiments, the first register address included in the instruction may comprise N number of bits of the instruction. A register corresponding to the first register address may include 2^(N) register entries, such that the first register address of the instruction points to a particular register entry of the register. Each register entry may comprise X number of bits, and the second register address may be determined based at least in part on the register entry pointed to by the first register address. A register corresponding to the second register address may include 2^(X) bits, such that the second register address may point to a particular bit location of the register corresponding to the second register address. Based at least in part on the value of the bit location pointed to by the second register address, the instruction may be predicated.

For example, a 32 bit instruction may include two bits for the first register address, such that the first register address points to one of four register entries. In this example, predication logic of a processor executing the instruction may access a particular register entry based on the two bit first register address. The bits stored at the particular register entry may correspond to the second register address. In this example, if each register entry stores six bits, the second register address may comprise six bits and may therefore point to a particular bit location of a sixty four bit register. As such, the predication logic may access the particular bit location corresponding to the second register address, and the predication logic may determine whether to predicate the instruction based on the value stored at the particular bit location.

As illustrated by the example provided above, the number of bits of an instruction corresponding to a predication register address may be reduced by pointing to a particular register entry, where each register entry may store a larger quantity of bits that may be used to point to a second register address location, including for example, a bit location of a predication register. Therefore, embodiments of the invention may reduce the number of bits of an instruction dedicated to indicating a predication register address relative to conventional processing architectures employing instruction predication.

Hardware and Software Environment

Now turning to the drawings, wherein like numbers denote like parts throughout the several views, FIG. 1 illustrates exemplary automated computing machinery including an exemplary computer 10 useful in data processing consistent with embodiments of the present invention. Computer 10 of FIG. 1 includes at least one computer processor 12 or ‘CPU’ as well as random access memory 14 (‘RAM’), which is connected through a high speed memory bus 16 and bus adapter 18 to processor 12 and to other components of the computer 10.

Stored in RAM 14 is an application program 20, a module of user-level computer program instructions for carrying out particular data processing tasks such as, for example, word processing, spreadsheets, database operations, video gaming, stock market simulations, atomic quantum process simulations, or other user-level applications. Also stored in RAM 14 is an operating system 22. Operating systems useful in connection with embodiments of the invention include UNIX™, Linux™, Microsoft Windows XP™, AIX™, IBM's i5/OS™, and others as will occur to those of skill in the art. Operating system 22 and application 20 in the example of FIG. 1 are shown in RAM 14, but many components of such software typically are stored in non-volatile memory also, e.g., on a disk drive 24.

As will become more apparent below, embodiments consistent with the invention may be implemented within Network On Chip (NOC) integrated circuit devices, or chips, and as such, computer 10 is illustrated including two exemplary NOCs: a video adapter 26 and a coprocessor 28. NOC video adapter 26, which may alternatively be referred to as a graphics adapter, is an example of an I/O adapter specially designed for graphic output to a display device 30 such as a display screen or computer monitor. NOC video adapter 26 is connected to processor 12 through a high speed video bus 32, bus adapter 18, and the front side bus 34, which is also a high speed bus. NOC Coprocessor 28 is connected to processor 12 through bus adapter 18, and front side buses 34 and 36, which is also a high speed bus. The NOC coprocessor of FIG. 1 may be optimized, for example, to accelerate particular data processing tasks at the behest of the main processor 12.

The exemplary NOC video adapter 26 and NOC coprocessor 28 of FIG. 1 each include a NOC, including integrated processor (‘IP’) blocks, routers, memory communications controllers, and network interface controllers, the details of which will be discussed in greater detail below in connection with FIGS. 2-3. The NOC video adapter and NOC coprocessor are each optimized for programs that use parallel processing and also require fast random access to shared memory. It will be appreciated by one of ordinary skill in the art having the benefit of the instant disclosure, however, that the invention may be implemented in devices and device architectures other than NOC devices and device architectures. The invention is therefore not limited to implementation within an NOC device.

Computer 10 of FIG. 1 includes disk drive adapter 38 coupled through an expansion bus 40 and bus adapter 18 to processor 12 and other components of the computer 10. Disk drive adapter 38 connects non-volatile data storage to the computer 10 in the form of disk drive 24, and may be implemented, for example, using Integrated Drive Electronics (‘IDE’) adapters, Small Computer System Interface (‘SCSI’) adapters, and others as will occur to those of skill in the art. Non-volatile computer memory also may be implemented for as an optical disk drive, electrically erasable programmable read-only memory (so-called ‘EEPROM’ or ‘Flash’ memory), RAM drives, and so on, as will occur to those of skill in the art.

Computer 10 also includes one or more input/output (‘I/O’) adapters 42, which implement user-oriented input/output through, for example, software drivers and computer hardware for controlling output to display devices such as computer display screens, as well as user input from user input devices 44 such as keyboards and mice. In addition, computer 10 includes a communications adapter 46 for data communications with other computers 48 and for data communications with a data communications network 50. Such data communications may be carried out serially through RS-232 connections, through external buses such as a Universal Serial Bus (‘USB’), through data communications data communications networks such as IP data communications networks, and in other ways as will occur to those of skill in the art. Communications adapters implement the hardware level of data communications through which one computer sends data communications to another computer, directly or through a data communications network. Examples of communications adapters suitable for use in computer 10 include modems for wired dial-up communications, Ethernet (IEEE 802.3) adapters for wired data communications network communications, and 802.11 adapters for wireless data communications network communications.

For further explanation, FIG. 2 sets forth a functional block diagram of an example NOC 102 according to embodiments of the present invention. The NOC in FIG. 2 is implemented on a ‘chip’ 100, that is, on an integrated circuit. NOC 102 includes integrated processor (‘IP’) blocks 104, routers 110, memory communications controllers 106, and network interface controllers 108 grouped into interconnected nodes. Each IP block 104 is adapted to a router 110 through a memory communications controller 106 and a network interface controller 108. Each memory communications controller controls communications between an IP block and memory, and each network interface controller 108 controls inter-IP block communications through routers 110.

In NOC 102, each IP block represents a reusable unit of synchronous or asynchronous logic design used as a building block for data processing within the NOC. The term ‘IP block’ is sometimes expanded as ‘intellectual property block,’ effectively designating an IP block as a design that is owned by a party, that is the intellectual property of a party, to be licensed to other users or designers of semiconductor circuits. In the scope of the present invention, however, there is no requirement that IP blocks be subject to any particular ownership, so the term is always expanded in this specification as ‘integrated processor block.’ IP blocks, as specified here, are reusable units of logic, cell, or chip layout design that may or may not be the subject of intellectual property. IP blocks are logic cores that can be formed as ASIC chip designs or FPGA logic designs.

One way to describe IP blocks by analogy is that IP blocks are for NOC design what a library is for computer programming or a discrete integrated circuit component is for printed circuit board design. In NOCs consistent with embodiments of the present invention, IP blocks may be implemented as generic gate netlists, as complete special purpose or general purpose microprocessors, or in other ways as may occur to those of skill in the art. A netlist is a Boolean-algebra representation (gates, standard cells) of an IP block's logical-function, analogous to an assembly-code listing for a high-level program application. NOCs also may be implemented, for example, in synthesizable form, described in a hardware description language such as Verilog or VHDL. In addition to netlist and synthesizable implementation, NOCs also may be delivered in lower-level, physical descriptions. Analog IP block elements such as SERDES, PLL, DAC, ADC, and so on, may be distributed in a transistor-layout format such as GDSII. Digital elements of IP blocks are sometimes offered in layout format as well. It will also be appreciated that IP blocks, as well as other logic circuitry implemented consistent with the invention may be distributed in the form of computer data files, e.g., logic definition program code, that define at various levels of detail the functionality and/or layout of the circuit arrangements implementing such logic. Thus, while the invention has and hereinafter will be described in the context of circuit arrangements implemented in fully functioning integrated circuit devices, data processing systems utilizing such devices, and other tangible, physical hardware circuits, those of ordinary skill in the art having the benefit of the instant disclosure will appreciate that the invention may also be implemented within a program product, and that the invention applies equally regardless of the particular type of computer readable storage medium being used to distribute the program product. Examples of computer readable storage media include, but are not limited to, physical, recordable type media such as volatile and non-volatile memory devices, floppy disks, hard disk drives, CD-ROMs, and DVDs (among others).

Each IP block 104 in the example of FIG. 2 is adapted to a router 110 through a memory communications controller 106. Each memory communication controller is an aggregation of synchronous and asynchronous logic circuitry adapted to provide data communications between an IP block and memory. Examples of such communications between IP blocks and memory include memory load instructions and memory store instructions. The memory communications controllers 106 are described in more detail below with reference to FIG. 3. Each IP block 104 is also adapted to a router 110 through a network interface controller 108, which controls communications through routers 110 between IP blocks 104. Examples of communications between IP blocks include messages carrying data and instructions for processing the data among IP blocks in parallel applications and in pipelined applications. The network interface controllers 108 are also described in more detail below with reference to FIG. 3.

Routers 110, and the corresponding links 118 therebetween, implement the network operations of the NOC. The links 118 may be packet structures implemented on physical, parallel wire buses connecting all the routers. That is, each link may be implemented on a wire bus wide enough to accommodate simultaneously an entire data switching packet, including all header information and payload data. If a packet structure includes 64 bytes, for example, including an eight byte header and 56 bytes of payload data, then the wire bus subtending each link is 64 bytes wide, 512 wires. In addition, each link may be bi-directional, so that if the link packet structure includes 64 bytes, the wire bus actually contains 1024 wires between each router and each of its neighbors in the network. In such an implementation, a message could include more than one packet, but each packet would fit precisely onto the width of the wire bus. In the alternative, a link may be implemented on a wire bus that is only wide enough to accommodate a portion of a packet, such that a packet would be broken up into multiple beats, e.g., so that if a link is implemented as 16 bytes in width, or 128 wires, a 64 byte packet could be broken into four beats. It will be appreciated that different implementations may used different bus widths based on practical physical limits as well as desired performance characteristics. If the connection between the router and each section of wire bus is referred to as a port, then each router includes five ports, one for each of four directions of data transmission on the network and a fifth port for adapting the router to a particular IP block through a memory communications controller and a network interface controller.

Each memory communications controller 106 controls communications between an IP block and memory. Memory can include off-chip main RAM 112, memory 114 connected directly to an IP block through a memory communications controller 106, on-chip memory enabled as an IP block 116, and on-chip caches. In NOC 102, either of the on-chip memories 114, 116, for example, may be implemented as on-chip cache memory. All these forms of memory can be disposed in the same address space, physical addresses or virtual addresses, true even for the memory attached directly to an IP block. Memory addressed messages therefore can be entirely bidirectional with respect to IP blocks, because such memory can be addressed directly from any IP block anywhere on the network. Memory 116 on an IP block can be addressed from that IP block or from any other IP block in the NOC. Memory 114 attached directly to a memory communication controller can be addressed by the IP block that is adapted to the network by that memory communication controller—and can also be addressed from any other IP block anywhere in the NOC.

NOC 102 includes two memory management units (‘MMUs’) 120, 122, illustrating two alternative memory architectures for NOCs consistent with embodiments of the present invention. MMU 120 is implemented within an IP block, allowing a processor within the IP block to operate in virtual memory while allowing the entire remaining architecture of the NOC to operate in a physical memory address space. MMU 122 is implemented off-chip, connected to the NOC through a data communications port 124. The port 124 includes the pins and other interconnections required to conduct signals between the NOC and the MMU, as well as sufficient intelligence to convert message packets from the NOC packet format to the bus format required by the external MMU 122. The external location of the MMU means that all processors in all IP blocks of the NOC can operate in virtual memory address space, with all conversions to physical addresses of the off-chip memory handled by the off-chip MMU 122.

In addition to the two memory architectures illustrated by use of the MMUs 120, 122, data communications port 126 illustrates a third memory architecture useful in NOCs capable of being utilized in embodiments of the present invention. Port 126 provides a direct connection between an IP block 104 of the NOC 102 and off-chip memory 112. With no MMU in the processing path, this architecture provides utilization of a physical address space by all the IP blocks of the NOC. In sharing the address space bi-directionally, all the IP blocks of the NOC can access memory in the address space by memory-addressed messages, including loads and stores, directed through the IP block connected directly to the port 126. The port 126 includes the pins and other interconnections required to conduct signals between the NOC and the off-chip memory 112, as well as sufficient intelligence to convert message packets from the NOC packet format to the bus format required by the off-chip memory 112.

In the example of FIG. 2, one of the IP blocks is designated a host interface processor 128. A host interface processor 128 provides an interface between the NOC and a host computer 10 in which the NOC may be installed and also provides data processing services to the other IP blocks on the NOC, including, for example, receiving and dispatching among the IP blocks of the NOC data processing requests from the host computer. A NOC may, for example, implement a video graphics adapter 26 or a coprocessor 28 on a larger computer 10 as described above with reference to FIG. 1. In the example of FIG. 2, the host interface processor 128 is connected to the larger host computer through a data communications port 130. The port 130 includes the pins and other interconnections required to conduct signals between the NOC and the host computer, as well as sufficient intelligence to convert message packets from the NOC to the bus format required by the host computer 10. In the example of the NOC coprocessor in the computer of FIG. 1, such a port would provide data communications format translation between the link structure of the NOC coprocessor 28 and the protocol required for the front side bus 36 between the NOC coprocessor 28 and the bus adapter 18.

FIG. 3 next illustrates a functional block diagram illustrating in greater detail the components implemented within an IP block 104, memory communications controller 106, network interface controller 108 and router 110 in NOC 102, collectively illustrated at 132 which may be referred to as a node or a hardware thread. IP block 104 includes a computer processor 134 and I/O functionality 136. In this example, computer memory is represented by a segment of random access memory (‘RAM’) 138 in IP block 104. The memory, as described above with reference to FIG. 2, can occupy segments of a physical address space whose contents on each IP block are addressable and accessible from any IP block in the NOC. The processors 134, I/O capabilities 136, and memory 138 in each IP block effectively implement the IP blocks as generally programmable microcomputers. As explained above, however, in the scope of the present invention, IP blocks generally represent reusable units of synchronous or asynchronous logic used as building blocks for data processing within a NOC. Implementing IP blocks as generally programmable microcomputers, therefore, although a common embodiment useful for purposes of explanation, is not a limitation of the present invention.

In NOC 102 of FIG. 3, each memory communications controller 106 includes a plurality of memory communications execution engines 140. Each memory communications execution engine 140 is enabled to execute memory communications instructions from an IP block 104, including bidirectional memory communications instruction flow 141, 142, 144 between the network and the IP block 104. The memory communications instructions executed by the memory communications controller may originate, not only from the IP block adapted to a router through a particular memory communications controller, but also from any IP block 104 anywhere in NOC 102. That is, any IP block in the NOC can generate a memory communications instruction and transmit that memory communications instruction through the routers of the NOC to another memory communications controller associated with another IP block for execution of that memory communications instruction. Such memory communications instructions can include, for example, translation lookaside buffer control instructions, cache control instructions, barrier instructions, and memory load and store instructions.

Each memory communications execution engine 140 is enabled to execute a complete memory communications instruction separately and in parallel with other memory communications execution engines. The memory communications execution engines implement a scalable memory transaction processor optimized for concurrent throughput of memory communications instructions. Memory communications controller 106 supports multiple memory communications execution engines 140 all of which run concurrently for simultaneous execution of multiple memory communications instructions. A new memory communications instruction is allocated by the memory communications controller 106 to a memory communications engine 140 and memory communications execution engines 140 can accept multiple response events simultaneously. In this example, all of the memory communications execution engines 140 are identical. Scaling the number of memory communications instructions that can be handled simultaneously by a memory communications controller 106, therefore, is implemented by scaling the number of memory communications execution engines 140.

In NOC 102 of FIG. 3, each network interface controller 108 is enabled to convert communications instructions from command format to network packet format for transmission among the IP blocks 104 through routers 110. The communications instructions may be formulated in command format by the IP block 104 or by memory communications controller 106 and provided to the network interface controller 108 in command format. The command format may be a native format that conforms to architectural register files of IP block 104 and memory communications controller 106. The network packet format is typically the format required for transmission through routers 110 of the network. Each such message is composed of one or more network packets. Examples of such communications instructions that are converted from command format to packet format in the network interface controller include memory load instructions and memory store instructions between IP blocks and memory. Such communications instructions may also include communications instructions that send messages among IP blocks carrying data and instructions for processing the data among IP blocks in parallel applications and in pipelined applications.

In NOC 102 of FIG. 3, each IP block is enabled to send memory-address-based communications to and from memory through the IP block's memory communications controller and then also through its network interface controller to the network. A memory-address-based communications is a memory access instruction, such as a load instruction or a store instruction, that is executed by a memory communication execution engine of a memory communications controller of an IP block. Such memory-address-based communications typically originate in an IP block, formulated in command format, and handed off to a memory communications controller for execution.

Many memory-address-based communications are executed with message traffic, because any memory to be accessed may be located anywhere in the physical memory address space, on-chip or off-chip, directly attached to any memory communications controller in the NOC, or ultimately accessed through any IP block of the NOC—regardless of which IP block originated any particular memory-address-based communication. Thus, in NOC 102, all memory-address-based communications that are executed with message traffic are passed from the memory communications controller to an associated network interface controller for conversion from command format to packet format and transmission through the network in a message. In converting to packet format, the network interface controller also identifies a network address for the packet in dependence upon the memory address or addresses to be accessed by a memory-address-based communication. Memory address based messages are addressed with memory addresses. Each memory address is mapped by the network interface controllers to a network address, typically the network location of a memory communications controller responsible for some range of physical memory addresses. The network location of a memory communication controller 106 is naturally also the network location of that memory communication controller's associated router 110, network interface controller 108, and IP block 104. The instruction conversion logic 150 within each network interface controller is capable of converting memory addresses to network addresses for purposes of transmitting memory-address-based communications through routers of a NOC.

Upon receiving message traffic from routers 110 of the network, each network interface controller 108 inspects each packet for memory instructions. Each packet containing a memory instruction is handed to the memory communications controller 106 associated with the receiving network interface controller, which executes the memory instruction before sending the remaining payload of the packet to the IP block for further processing. In this way, memory contents are always prepared to support data processing by an IP block before the IP block begins execution of instructions from a message that depend upon particular memory content.

In NOC 102 of FIG. 3, each IP block 104 is enabled to bypass its memory communications controller 106 and send inter-IP block, network-addressed communications 146 directly to the network through the IP block's network interface controller 108. Network-addressed communications are messages directed by a network address to another IP block. Such messages transmit working data in pipelined applications, multiple data for single program processing among IP blocks in a SIMD application, and so on, as will occur to those of skill in the art. Such messages are distinct from memory-address-based communications in that they are network addressed from the start, by the originating IP block which knows the network address to which the message is to be directed through routers of the NOC. Such network-addressed communications are passed by the IP block through I/O functions 136 directly to the IP block's network interface controller in command format, then converted to packet format by the network interface controller and transmitted through routers of the NOC to another IP block. Such network-addressed communications 146 are bi-directional, potentially proceeding to and from each IP block of the NOC, depending on their use in any particular application. Each network interface controller, however, is enabled to both send and receive such communications to and from an associated router, and each network interface controller is enabled to both send and receive such communications directly to and from an associated IP block, bypassing an associated memory communications controller 106.

Each network interface controller 108 in the example of FIG. 3 is also enabled to implement virtual channels on the network, characterizing network packets by type. Each network interface controller 108 includes virtual channel implementation logic 148 that classifies each communication instruction by type and records the type of instruction in a field of the network packet format before handing off the instruction in packet form to a router 110 for transmission on the NOC. Examples of communication instruction types include inter-IP block network-address-based messages, request messages, responses to request messages, invalidate messages directed to caches; memory load and store messages; and responses to memory load messages, etc.

Each router 110 in the example of FIG. 3 includes routing logic 152, virtual channel control logic 154, and virtual channel buffers 156. The routing logic typically is implemented as a network of synchronous and asynchronous logic that implements a data communications protocol stack for data communication in the network formed by the routers 110, links 118, and bus wires among the routers. Routing logic 152 includes the functionality that readers of skill in the art might associate in off-chip networks with routing tables, routing tables in at least some embodiments being considered too slow and cumbersome for use in a NOC. Routing logic implemented as a network of synchronous and asynchronous logic can be configured to make routing decisions as fast as a single clock cycle. The routing logic in this example routes packets by selecting a port for forwarding each packet received in a router. Each packet contains a network address to which the packet is to be routed.

In describing memory-address-based communications above, each memory address was described as mapped by network interface controllers to a network address, a network location of a memory communications controller. The network location of a memory communication controller 106 is naturally also the network location of that memory communication controller's associated router 110, network interface controller 108, and IP block 104. In inter-IP block, or network-address-based communications, therefore, it is also typical for application-level data processing to view network addresses as the location of an IP block within the network formed by the routers, links, and bus wires of the NOC. FIG. 2 illustrates that one organization of such a network is a mesh of rows and columns in which each network address can be implemented, for example, as either a unique identifier for each set of associated router, IP block, memory communications controller, and network interface controller of the mesh or x, y coordinates of each such set in the mesh.

In NOC 102 of FIG. 3, each router 110 implements two or more virtual communications channels, where each virtual communications channel is characterized by a communication type. Communication instruction types, and therefore virtual channel types, include those mentioned above: inter-IP block network-address-based messages, request messages, responses to request messages, invalidate messages directed to caches; memory load and store messages; and responses to memory load messages, and so on. In support of virtual channels, each router 110 in the example of FIG. 3 also includes virtual channel control logic 154 and virtual channel buffers 156. The virtual channel control logic 154 examines each received packet for its assigned communications type and places each packet in an outgoing virtual channel buffer for that communications type for transmission through a port to a neighboring router on the NOC.

Each virtual channel buffer 156 has finite storage space. When many packets are received in a short period of time, a virtual channel buffer can fill up—so that no more packets can be put in the buffer. In other protocols, packets arriving on a virtual channel whose buffer is full would be dropped. Each virtual channel buffer 156 in this example, however, is enabled with control signals of the bus wires to advise surrounding routers through the virtual channel control logic to suspend transmission in a virtual channel, that is, suspend transmission of packets of a particular communications type. When one virtual channel is so suspended, all other virtual channels are unaffected—and can continue to operate at full capacity. The control signals are wired all the way back through each router to each router's associated network interface controller 108. Each network interface controller is configured to, upon receipt of such a signal, refuse to accept, from its associated memory communications controller 106 or from its associated IP block 104, communications instructions for the suspended virtual channel. In this way, suspension of a virtual channel affects all the hardware that implements the virtual channel, all the way back up to the originating IP blocks.

One effect of suspending packet transmissions in a virtual channel is that no packets are ever dropped. When a router encounters a situation in which a packet might be dropped in some unreliable protocol such as, for example, the Internet Protocol, the routers in the example of FIG. 3 may suspend by their virtual channel buffers 156 and their virtual channel control logic 154 all transmissions of packets in a virtual channel until buffer space is again available, eliminating any need to drop packets. The NOC of FIG. 3, therefore, may implement highly reliable network communications protocols with an extremely thin layer of hardware.

The example NOC of FIG. 3 may also be configured to maintain cache coherency between both on-chip and off-chip memory caches. Each NOC can support multiple caches each of which operates against the same underlying memory address space. For example, caches may be controlled by IP blocks, by memory communications controllers, or by cache controllers external to the NOC. Either of the on-chip memories 114, 116 in the example of FIG. 2 may also be implemented as an on-chip cache, and, within the scope of the present invention, cache memory can be implemented off-chip also.

Each router 110 illustrated in FIG. 3 includes five ports, four ports 158A-D connected through bus wires 118 to other routers and a fifth port 160 connecting each router to its associated IP block 104 through a network interface controller 108 and a memory communications controller 106. As can be seen from the illustrations in FIGS. 2 and 3, the routers 110 and the links 118 of the NOC 102 form a mesh network with vertical and horizontal links connecting vertical and horizontal ports in each router. In the illustration of FIG. 3, for example, ports 158A, 158C and 160 are termed vertical ports, and ports 158B and 158D are termed horizontal ports.

FIG. 4 next illustrates in another manner one exemplary implementation of an IP block 104 consistent with the invention, implemented as a processing element partitioned into an instruction unit (IU) 162, execution unit (XU) 164 and auxiliary execution unit (AXU) 166. In the illustrated implementation, IU 162 includes a plurality of instruction buffers 168 that receive instructions from an L1 instruction cache (iCACHE) 170. Each instruction buffer 168 is dedicated to one of a plurality, e.g., four, symmetric multithreaded (SMT) hardware threads. An effective-to-real translation unit (iERAT) 172 is coupled to iCACHE 170, and is used to translate instruction fetch requests from a plurality of thread fetch sequencers 174 into real addresses for retrieval of instructions from lower order memory. Each thread fetch sequencer 174 is dedicated to a particular hardware thread, and is used to ensure that instructions to be executed by the associated thread is fetched into the iCACHE for dispatch to the appropriate execution unit. As also shown in FIG. 4, instructions fetched into instruction buffer 168 may also be monitored by branch prediction logic 176, which provides hints to each thread fetch sequencer 174 to minimize instruction cache misses resulting from branches in executing threads.

IU 162 also includes a dependency/issue logic block 178 dedicated to each hardware thread, and configured to resolve dependencies and control the issue of instructions from instruction buffer 168 to XU 164. In addition, in the illustrated embodiment, separate dependency/issue logic 180 is provided in AXU 166, thus enabling separate instructions to be concurrently issued by different threads to XU 164 and AXU 166. In an alternative embodiment, logic 180 may be disposed in IU 162, or may be omitted in its entirety, such that logic 178 issues instructions to AXU 166.

XU 164 is implemented as a fixed point execution unit, including a set of general purpose registers (GPR's) 182 coupled to fixed point logic 184, branch logic 186 and load/store logic 188. Load/store logic 188 is coupled to an L1 data cache (dCACHE) 190, with effective to real translation provided by dERAT logic 192. XU 164 may be configured to implement practically any instruction set, e.g., all or a portion of a 32b or 64b PowerPC instruction set.

AXU 166 operates as an auxiliary execution unit including dedicated dependency/issue logic 180 along with one or more execution blocks 194. AXU 166 may include any number of execution blocks, and may implement practically any type of execution unit, e.g., a floating point unit, or one or more specialized execution units such as encryption/decryption units, coprocessors, vector processing units, graphics processing units, XML processing units, etc. In the illustrated embodiment, AXU 166 includes a high speed auxiliary interface to XU 164, e.g., to support direct moves between AXU architected state and XU architected state.

Communication with IP block 104 may be managed in the manner discussed above in connection with FIG. 2, via network interface controller 108 coupled to NOC 102. Address-based communication, e.g., to access L2 cache memory, may be provided, along with message-based communication. For example, each IP block 104 may include a dedicated in box and/or out box in order to handle inter-node communications between IP blocks. Embodiments of the present invention may be implemented within the hardware and software environment described above in connection with FIGS. 1-4. However, it will be appreciated by one of ordinary skill in the art having the benefit of the instant disclosure that the invention may be implemented in a multitude of different environments, and that other modifications may be made to the aforementioned hardware and software embodiment without departing from the spirit and scope of the invention. As such, the invention is not limited to the particular hardware and software environment disclosed herein.

Instruction Predication Utilizing an Indirect Predication Register

In some embodiments of the invention an instruction may include a first register address. Predication logic of a processing unit may determine a second register address based at least in part on the value stored at the first register address, and the predication logic may determine whether to predicate the instruction based at least in part on the value stored at the second register address.

In some embodiments, predication logic of a processing unit may determine a bit location of a predication register that indicates whether to predicate an instruction based on a value stored at a register entry of an indirect predication register. The address of the relevant register entry of the indirect predication register may be included in the instruction. The number of bits of the instruction dedicated to pointing to the register entry of the indirect predication register may be less than the number of bits needed to point to the bit location of the predication register. As such, bit space may be saved in the instruction by pointing to the register entry of the indirect predication register as compared to directly pointing to the bit location of the predication register.

Turning now to FIG. 5, this figure illustrates an exemplary block diagram of a processor 200 or processing unit consistent with embodiments of the invention including decode logic 202 for decoding received instructions for execution by an execution unit 204 of the processor 200. In addition, processor 200 includes predication logic 206 coupled to the instruction decode logic 202, where predication logic 206 is configured to analyze instructions in an instruction stream in parallel with instructions being decoded by the decode logic 202 to determine whether to predicate each instruction in the instruction stream. In some embodiments of the invention, the predication logic 206 determines a first register address from an instruction, determines a second register address based on a value stored at the first register address, and determines whether to predicate the instruction based on a value stored at the second register address.

Processor 200 also includes issue/dependency logic 214, where issue/dependency logic 214 issues instructions to the execution unit 204 for execution. The predication logic 206 may communicate a predication signal to the dependency/issue logic 214 and/or execution unit 204 indicating whether to execute or predicate an instruction in the instruction stream. As shown, instructions of the instruction stream are loaded into the decode logic 202 and the predication logic 206 substantially in parallel, and data may be communicated from the decode logic 202 to the predication logic 206, including for example a signal indicating whether a loaded instruction is a valid instruction and/or data corresponding to decoding a loaded instruction.

FIG. 6 is a block diagram of an example implementation of predication logic 206 of the processor 200 of FIG. 5. The predication logic may determine a first register address corresponding to a register entry of an indirect predicate register 220 from an instruction. As shown, the indirect predicate register address may comprise N bits of the instruction, and the indirect predicate register may include 2^(N) register entries. Therefore, in some embodiments the N bits of the indirect predicate register address of the instruction may point to the 2^(N) register entries of the indirect predicate register.

A value stored at the indirect predicate register entry pointed to by the indirect predicate register address of the instruction is used to determine a second register address corresponding to a direct predicate register 222. In some embodiments, each indirect predicate register 220 entry stores X bits of data, and the predicate direct register 222 stores 2^(X) bits. Therefore, in some embodiments, each register entry of the indirect predicate register 220 may point to direct predicate register bit address of the direct predicate register 222. As shown in FIG. 6, a value stored at the direct predicate register bit address of the of the direct predicate register 222 may be logically combined with an instruction valid signal received from instruction decode logic associated with the predication logic 206 to output a predication signal that indicates whether to predicate the instruction.

Turning now to FIG. 7, this figure provides a flowchart 300 that illustrates a sequence of operations that may be performed by a processor consistent with embodiments of the invention to selectively predicate an instruction. A first register address is determined based at least in part on the instruction (block 302). In some embodiments, the instruction may include one or more bits that indicate a register address. In some embodiments, one or more of the bits that indicate a register address may also correspond to other portions of the instruction, including for example a primary opcode portion, a secondary opcode portion, a source address, and/or a target address. As such, in these embodiments, predication logic of the processor may determine the first register address by analyzing the one or more bits that indicate a register address.

The predication logic may access a register corresponding to the first register address to determine a value stored at the first register address, and based on the value stored at the first register address, the predication logic may determine a second register address (block 304). In some embodiments, the first register address may point to a particular register entry of a plurality of possible register entries, and the value at the particular register entry may correspond to the second register address. For example, in some embodiments, the particular register entry may store at least a portion of the second register address. The predication logic may access a register corresponding to the second register address to determine a value stored at the second register address, and the processor may selectively predicate the instruction based at least in part on the value stored at the second register address (block 306).

FIG. 8 provides flowchart 320 that illustrates a sequence of operations that may be performed by a processor consistent with embodiments of the invention to selectively predicate instructions in an instruction stream. An instruction of the instruction stream is received at the processor (block 322). As shown above with respect to FIG. 5, the processor may receive an instruction concurrently at an instruction decode logic and a predication logic of the processor. The processor determines whether the instruction is of a type that supports instruction predication (block 324). In some embodiments, instruction decode logic of the processor may decode the received instruction and determine whether the instruction is of the type that supports instruction predication, and the instruction decode logic may communicate a signal to the predication logic indicating whether the instruction is of the type that supports predication. In response to determining that the instruction is not of the type that supports predication (“N” branch of block 324), the processor may execute the instruction (block 326).

In response to determining that the instruction is of the type that supports predication (“Y” branch of block 324), a first register address may be determined based at least in part on the instruction (block 328). In some embodiments the instruction may include one or more bits that indicate the first register address. In some embodiments the predication logic may analyze such bits to determine the first register address. In some embodiments, the instruction decode logic may communicate the bits indicating the first register address to the predication logic. A register corresponding to the first register address is accessed to determine a value stored at the first register address, and the predication logic determines a second register address based at least in part on the value stored at the first register address (block 330).

A register corresponding to the second register address is accessed to determine a value stored at the second register address (block 332), and the processor determines whether to predicate the instruction based at least in part on the value stored at the second register address (block 334). In response to determining to predicate the instruction (“Y” branch of block 334), the instruction is predicated and a next instruction in the instruction stream is received (block 322). In response to determining not to predicate the instruction (“N” branch of block 334), the processor executes the instruction (block 336).

As such, in embodiments of the invention performing operations consistent with flowchart 320, the first register address may be determined based on the instruction, and a second register address may be determined based on the value stored at the first register address. The instruction may be selectively predicated based on the value stored at the second register address. Moreover, in these embodiments, utilization of the first register address that stores a value corresponding to the second register address may reduce the number of bits in each instruction utilized to point to a predication register address. In embodiments of the invention, a register corresponding to the first register address (e.g., an indirect predicate register) and a register corresponding to the second register address (e.g., a direct predicate register) may be fully architected such that move to/from general purpose register (GPR) type instructions may be supported.

To illustrate an example application instruction predication, a pixel shader code example is provided below including a plurality of instructions in an instruction stream that may be selectively predicated. In the first example, Example 1, the pixel shader code does not utilize indirect instruction predication. In the second example, Example 2, the pixel shader code utilizes indirect instruction predication consistent with embodiments of the invention. The pseudo-code examples provided below provide a portion of an unrolled loop performing triangle rasterization. The examples perform the following tasks in support of rasterizing 4 pixels in succession: load the previous Z buffer value for that location, calculate the barycentric coordinates, calculate the Z depth for the new pixel, check if Z buffer testing is enabled (zflag=1) and if so, check to see if the new pixel is closer to the camera than the old one. If it is, update the Z buffer and draw the pixel. Otherwise, skip on to the next pixel.

Example 1

pixel0: lfsx prev_z0, r_zb, # load prev z value r_i vaddfp bcc, bcc, bcci # update barycentric coords vdot3fp new_z0, bcc, z0 # calculate z for new pixel cmpi zflag, 1 bneq n0 fcmp prev_z0, # compare z values new_z0 bge pixel1 # skip if new pixel is behind a prev drawn one n0: stfsx new_z0, r_zb, # store to zbuffer (if not r_i predicated) stvx color_z0, r_cb, # store the color to the color r_i buffer addi r_i, r_i, 4 # update the zbuffer pointer pixel1: lfsx prev_z1, r_zb, # load prev z value r_i vaddfp bcc, bcc, bcci # update barycentric coords vdot3fp new_z1, bcc, z1 # calculate z for new pixel cmpi zflag, 1 bneq n1 fcmp prev_z1, # compare z values new_z1 bge pixel2 # skip if new pixel is behind a prev drawn one n1: stfsx new_z1, r_zb, # store to zbuffer (if not r_i predicated) stvx color_z1, r_cb, # store the color to the color r_i buffer addi r_i, r_i, 4 # update the zbuffer pointer pixel2: lfsx prev_z2, r_zb, # load prev z value r_i vaddfp bcc, bcc, bcci # update barycentric coords vdot3fp new_z1, bcc, z2 # calculate z for new pixel cmpi zflag, 1 bneq n2 fcmp prev_z2, # compare z values new_z2 bge pixel3 # skip if new pixel is behind a prev drawn one n2: stfsx new_z2, r_zb, # store to zbuffer (if not r_i predicated) stvx color_z2, r_cb, # store the color to the color r_i buffer addi r_i, r_i, 4 # update the zbuffer pointer pixel3: lfsx prev_z3, r_zb, # load prev z value r_i vaddfp bcc, bcc, bcci # update barycentric coords vdot3fp new_z3, bcc, z3 # calculate z for new pixel cmpi zflag, 1 bneq n3 fcmp prev_z3, # compare z values new_z3 bge pixel3 # skip if new pixel is behind a prev drawn one n2: stfsx new_z3, r_zb, # store to zbuffer (if not r_i predicated) stvx color_z3, r_cb, # store the color to the color r_i buffer addi r_i, r_i, 4 # update the zbuffer pointer

In this example, the large number of inline branch mispredicts may hamper performance, as some of the branches may not be taken, which leads to pipeline flushes.

Example 2

cmpi zflag, 1 beq pixel0 #skip to the start if ztest is enabled ztest_disabled: mtip 1, 61 #move the value 61 to ind pred register 1 mfdptgpr 13 #copy the contents of direct pred register to GPR13 ori 13, 13, 0x4 #set bit 61 of GPR13 mfgprtdp 13 #copy GPR13 to the dir pred register pixel0: lfsx prev_z0, r_zb, # load prev z value r_i vaddfp bcc, bcc, bcci # update barycentric coords vdot3fp new_z0, bcc, z0 # calculate z for new pixel fcmp prev_z0, # compare z values new_z0 bge pixel1 # skip if new pixel is behind a prev drawn one stfsx new_z0, r_zb, # store to zbuffer (if not r_i, p1 predicated) stvx color_z0, r_cb, # store the color to the color r_i buffer addi r_i, r_i, 4, p1 # update the zbuffer pointer pixel1: lfsx prev_z1, r_zb, # load prev z value r_i vaddfp bcc, bcc, bcci # update barycentric coords vdot3fp new_z1, bcc, z1 # calculate z for new pixel fcmp prev_z1, # compare z values new_z1 bge pixel2 # skip if new pixel is behind a prev drawn one stfsx new_z1, r_zb, # store to zbuffer (if not r_i, p1 predicated) stvx color_z1, r_cb, # store the color to the color r_i buffer addi r_i, r_i, 4, p1 # update the zbuffer pointer pixel2: lfsx prev_z2, r_zb, # load prev z value r_i vaddfp bcc, bcc, bcci # update barycentric coords vdot3fp new_z2, bcc, z2 # calculate z for new pixel fcmp prev_z2, # compare z values new_z2 bge pixel3 # skip if new pixel is behind a prev drawn one stfsx new_z2, r_zb, # store to zbuffer (if not r_i, p1 predicated) stvx color_z2, r_cb, # store the color to the color r_i buffer addi r_i, r_i, 4, p1 # update the zbuffer pointer pixel3: lfsx prev_z3, r_zb, # load prev z value r_i vaddfp bcc, bcc, bcci # update barycentric coords vdot3fp new_z3, bcc, z3 # calculate z for new pixel fcmp prev_z3, # compare z values new_z3 bge pixel4 # skip if new pixel is behind a prev drawn one stfsx new_z3, r_zb, # store to zbuffer (if not r_i, p1 predicated) stvx color_z3, r_cb, # store the color to the color r_i buffer addi r_i, r_i, 4, p1 # update the zbuffer pointer

In this example, based on a compare instruction (i.e., comparing z values for each pixel), one or more instructions may be predicated in the instruction stream. Referring to the portion of code directed to ‘pixel0’, the instruction ‘fcmp prev_z0, new_z0’ compares an old z-value to a new z-value, and based on the compare, the four instructions following the compare instruction may be skipped (i.e., the new object will not be drawn for the pixel because a previously drawn object for the pixel is closer to the camera). As shown in the example, an indirect predicate register entry is set to the value 61 (‘mtip 1, 61’), and a bit location ‘61’ of a direct predication register is set to indicate whether to predicate the instruction (‘ori 13, 13, 0x4’; ‘mfgprtdp 13’).

FIG. 9 provides a block diagram illustrating a 32 bit instruction 400 including support for instruction predication. As shown, the 32 bit instruction includes 6 bits dedicated to a primary opcode 402 for the instruction, 5 bits dedicated to a target address 404, 5 bits dedicated to each of three source addresses (VA 406, VB 408, and VC 410), and 6 bits dedicated to a predication register address 412. In contrast, FIG. 10 provides a block diagram illustrating a 32 bit instruction 450 including support for indirect instruction predication. As shown, the instruction includes 6 bits dedicated to a primary opcode 452 for the instruction, 5 bits dedicated to a target address, 5 bits dedicated to each of three source addresses (VA 456, VB 458, and VC 460), 2 bits dedicated an indirect predication register address 462, and 4 bits dedicated to a secondary opcode for the instruction. As illustrated by the example, embodiments of the invention may reduce the number of bits in an instruction dedicated to predication such that bits may be dedicated to a secondary opcode.

While the invention has been illustrated by a description of the various embodiments and the examples, and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any other way limit the scope of the appended claims to such detail. For example, the blocks of any of the flowcharts may be re-ordered, processed serially and/or processed concurrently without departing from the scope of the invention. Moreover, any of the flowcharts may include more or fewer blocks than those illustrated consistent with embodiments of the invention. Additional advantages and modifications will readily appear to those skilled in the art. Thus, the invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. In particular, any of the blocks of the above flowcharts may be deleted, augmented, made to be simultaneous with another, combined, or be otherwise altered in accordance with the principles of the invention. Accordingly, departures may be made from such details without departing from the spirit or scope of applicants' general inventive concept. 

What is claimed is:
 1. A method of executing instructions in a processor, the method comprising: receiving an instruction including a first register address; determining a second register address based at least in part on a value stored at the first register address; and selectively predicating the instruction based at least in part on a value stored at the second register address.
 2. The method of claim 1, wherein the first register address corresponds to a first register and the second register address corresponds to a second register.
 3. The method of claim 1, wherein the value stored at the first register address corresponds to a bit address of the second register, and determining the second register address based at least in part on the value stored at the first register address comprises retrieving the value stored at the first register address.
 4. The method of claim 1, wherein the first register address comprises N bits of the instruction that points to one of 2^(N) register entries.
 5. The method of claim 1, wherein selectively predicating the instruction comprises communicating a predication enable signal to an execution unit of the processor, the predication enable signal indicating to predicate the instruction.
 6. The method of claim 5, wherein the predication enable signal is based on the value stored at the second register address.
 7. The method of claim 1, further comprising: determining whether the instruction is of a type that supports predication at decoding logic of the processor, wherein selectively predicating the instruction is based at least in part on the determination of whether the instruction is of a type that supports predication.
 8. The method of claim 1, wherein the value stored at the second register address is a bit value, and selectively predicating the instruction based at least in part on the value stored at the second register address comprises: predicating the instruction responsive to the bit value stored at the second register address having a value of one. 